Scuba Air Consumption Calculator
SAC rate and air volume use from tank pressures and depth.
What it does
Two divers can share a depth and a dive time yet surface with very different gauges. Fitness, trim, current, and cold all feed breathing rate, and the SAC number puts a figure on it that follows the diver from dive to dive.
The tool above takes start and end pressures, average depth, dive time, and cylinder size. It returns SAC with RMV volume rate beside them: SAC in gauge units per minute, RMV in gas volume per minute at the surface.
The defaults log an Aluminum 80 from 3000 to 1500 psi over 30 min at 33 ft: SAC 25 psi per min, RMV 0.65 cu ft per min. The metric mirror (200 to 100 bar, 10 m, 12 L) gives 1.67 bar per min with 20 L per min.
The method matches Divers Alert Network estimating guidance with NOAA pressure facts behind ambient math: pressure rises one atmosphere per 33 ft of seawater or 10 m. Both name the same divide gas by minutes by atmospheres line.
How to use
- Pick imperial (psi, feet) or metric (bar, meters).
- Enter start and end pressures.
- Enter average depth for the whole dive.
- Enter bottom time in minutes.
- Enter tank factor or cylinder liters.
- Read SAC first, RMV for other cylinders.
Formula
- ATA
- Ambient pressure in atmospheres absolute.
- SAC
- Gauge units per minute: psi/min or bar/min.
- RMV
- Gas volume per minute: cu ft/min or L/min.
- factor
- Cylinder volume over rated pressure (metric: liters).
Depth costs gas by Boyle law, so dividing atmospheres back out leaves a personal surface constant. Multilevel dives should use true average depth; fresh water runs about 34 ft per atmosphere instead of 33.
Worked example
Example: Al80, 3000 to 1500 psi, 33 ft, 30 min
Defaults: imperial, 3000 to 1500 psi, 33 ft average, 30 min, factor 0.0258.
- Used: 3000 - 1500 = 1500 psi.
- ATA: 1 + 33/33 = 2.
- SAC: (1500 / 30) / 2 = 25 psi per min.
- RMV: 25 x 0.0258 = 0.65 cu ft per min.
- Tool shows 25 psi per min, 0.65 cu ft per min, 1500 psi used.
- Metric mirror gives 1.67 bar per min, 20 L per min.
Reference table
| Cylinder | Math | Factor |
|---|---|---|
| Aluminum 80 | 77.4 cu ft / 3000 psi | 0.0258 |
| Metric 12 L | 12 L water volume | 12 |
| Your cylinder | Volume / rated psi | Do the division |
Limits
- Logs past breathing; never a gas plan.
- Average depth smooths multilevel profiles.
- Cold, current, and exertion move the rate.
- Gauge error and rounding ride along.
Common mistakes
- Carrying psi SAC to a different cylinder.
- Using max depth instead of average.
- Planning dives to the last breath.
- Forgetting fresh water runs shallower per ATA.
- Reading SAC once and trusting it forever.
Try it yourself
Quick checks. Answers sit tucked under each question.
3000 to 1000 psi, 40 ft, 40 min, Al80?
22.6 psi per min, 0.58 cu ft per min.
200 to 120 bar, 15 m, 40 min, 12 L?
0.8 bar per min, 9.6 L per min.
SAC or RMV for a new cylinder?
RMV travels; psi per min is tied to the old tank.
Questions
What is SAC rate?
Surface air consumption: breathing cost at the surface with depth divided out.
SAC vs RMV?
SAC runs in gauge units per minute; RMV runs in gas volume per minute and crosses cylinders.
What is a normal SAC?
Relaxed divers often sit near 0.5 to 0.7 cu ft per min; new divers run higher.
Why average depth?
Gas cost follows ambient pressure minute by minute, so the mean depth rules.
Fresh vs salt water?
Fresh needs about 34 ft per atmosphere against 33 in seawater.
Can I plan gas from SAC?
Roughly, but reserves, ascent, and safety stops need their own planning rules.